The present invention concerns improvements in microwave cavities constituting the body of heating devices more particularly intended for continuously heating elongate objects made of low dielectric loss material, such as silica and glass.
The constant development of telecommunications is naturally leading to the use of carrier waves of ever increasing frequencies. The use of optical-frequency waves has focussed the interest in light guides consisting of glass fibres and many works have already been published on this subject. The use of light guides as communication medium calls for the development of industrial processes for the manufacture of the fibre, as well as for the interconnection of the fibres. Research has already shown that the chemical composition of the fibre is basic. It is therefore essential to apply non-polluting processes for manufacturing and treating optical fibres. Microwave energy heating exhibits this quality. As compared with r.f. heating, it has the advantage that it can be focussed in space, so that it is possible, for example, to concentrate the available energy in a small volume and to reach a high energy density from a low consumed power despite the relatively poor yield of conversion of the mains energy into microwave energy. This feature is particularly interesting when the objects to be heated have small dimensions, at least in one direction. It will be recalled that the diameter of optical fibres employed in telecommunications is of the order of 150 micrometers at most and reaches several tens of microns in some designs, before sheathing.
Among the various cavities which can be used as microwave heating apparatus, the one most particularly adapted to the continuous heating of thin cylindrical objects is the cylindrical cavity excited in the TM.sub.010 mode. A study of such a cavity employed as a heating device has been made by METAXAS (See "Design of a TM.sub.010 Resonant Cavity as a Heating Device at 2.45 GHz " by A. C. METAXAS in Journal of Microwave Power 9(2), 1974, pp 123-128).
Some of results of this study are summed up hereunder: the electric field vector is parallel to the axis of the cylinder and its modulus is constant in a direction parallel to this axis and maximum in the neighbourhood of the said axis. The magnetic field lines are concentric circles whose plane is perpendicular to the axis and which are centred on the axis. This mode of excitation of the cavity therefore takes the axis of the cylinder as the axis of symmetry. The wavelength in the cavity depends only upon its radius and the energy density is maximum close to the axis. This cavity and this mode of excitation are therefore particularly well adapted to the continuous heating of cylindrical objects of small cross-section.